The ongoing miniaturization in integrated circuits with increased complexity and multilevel metal layers and the focus on increasing speed of these circuits demand for low permittivity materials, particularly for use as intermetal dielectric layers. Conventionally, metal interconnects, mostly aluminum layers, with silicon dioxide as intermetal dielectric are used, but this conventional solution will not be able to meet the stringent specifications resulting from the above mentioned trends. Therefore, to avoid that the larger portion of the total circuit delay is caused by the resistance and capacitance of the interconnect system, one has to reduce the permittivity of the dielectric used. This is stated in numerous publications, e.g. in Table 1 of R. K. Laxman, "Low .di-elect cons. dielectric: CVD Fluorinated Silicon Dioxides", Semiconductor International, May 1995, pp. 71-74. Therefore miniaturization has lead to an intensified search for new low K materials. A low .di-elect cons. material, a low K material and a material with a low permittivity are all alternative expressions for a material with a low dielectric constant, at least for the purposes of this disclosure. The most desirable material should have a low K value, low mechanical stress, high thermal stability and low moisture absorption. Furthermore, the desired material should be selected based on the compatibility with state-of-the-art semiconductor processing steps and tools.
Part of the search for new low K materials was directed to changing the properties of silicon dioxide as deposited. Deposited silicon dioxide is the most widely used intermetal dielectric material having a K value of about 3.9. Several publications have indicated that the K value of silicon dioxide films can be reduced by introducing increasing amounts of fluorine in said films. Fluorine is the most electronegative and the least polarizable element on the periodic table. Incorporation of fluorine reduces the number of polarizable Si-OH bonds and also influences the silicon oxide such that it has a less polarizable geometry to thereby lowering the K value of the fluorinated silicon oxide films. A wide variety of processes to deposit fluorinated silicon oxide films are known like e.g. a Plasma Enhanced Chemical Vapour Deposition (PECVD) process as in the U.S. Pat. No. 5,641,581. Using these processes K values in the range between 3 and 3.5 are reported dependent on the amount of fluorine atoms incorporated, i.e. an increasing amount of fluorine leads to a decrease of the K value.
Besides the focus on changing the properties of silicon oxide, there is an ongoing search for new low K materials. Among these new materials are the organic spin-on materials, having a K value in the range from 2.5 to 3, and the inorganic low-K materials as e.g. xerogels having a K value typically lower than 1.5. Many of these new low-K materials comprise fluorine. The organic materials are of particular interest because they feature simplified processing, excellent gap-fill and planarization. Furthermore the K-value of organic materials, which comprise Phenyl groups, can be additionally lowered by plasma fluorination as e.g. in H. Kudo et al., Mat. Res. Symp. Proc., Vol. 381, pp. 105-110, 1985. By doing so the K-values can be lowered yielding a range from 2 to 2.5 instead of from 2.5 to 3.
In summary, it is clear that fluorine-containing dielectrics have in general a lower K-value than there unfluorinated counterparts. This holds both for polymer like and ceramic like dielectrics. Therefore fluorine-containing dielectrics are of particular interest in order to avoid that the larger portion of the total circuit delay is caused by the capacitance of the interconnect system. Despite all these advantages fluorine-containing dielectrics are not compatible with current metallization structures using metallization materials such as Ti, or Ta, or W, or the nitrides of each of the aforementioned materials, or Cu, or Al. This is due to the fact that the incorporation of fluorine has been shown to be detrimental for the aforementioned metallization materials.